Preheat your oven to 350ºF (175ºC) and grease a 9×5 loaf pan with butter or oil.

In a large bowl, cream the softened butter using a hand mixer or stand mixer fitted with the paddle attachment until smooth.

Add in the granulated and brown sugars, mixing on medium-high speed until light and fluffy (about 2-3 minutes).

Incorporate the egg, vanilla extract, and buttermilk, mixing until well combined.

Sift together the flour and baking soda, then gently fold these dry ingredients into the wet mixture, being careful not to overmix.

In a separate bowl, mix together ⅓ cup sugar with 1 ½ teaspoons of cinnamon.

Pour half of the batter into your prepared loaf pan. Sprinkle half of the cinnamon sugar mixture over it and swirl with a butter knife.

Spread the remaining batter evenly on top, followed by the rest of the cinnamon sugar mixture. Swirl again to combine.

Drizzle 2 tablespoons of melted butter over the top.

Bake in your preheated oven for 50-70 minutes, or until a toothpick comes out clean when inserted into the center.

Let the bread cool in the pan for 20 minutes before transferring it to a wire rack to cool completely before slicing.